Modulation is the process by which the characteristics of the carrier wave is varied or
altered in accordance with the instantaneous amplitude of modulating signal. Modulating
signal, usually low frequency signal or audio frequency (A.F). signal means the signal to be
modulated, i.e. message signal. The message signal contains some useful information. The
carrier signal is (usually high frequency signal or radio frequency signal) used to carry the
modulating signal. Let a sinusoidal wave in analog modulation be given as
) sin( ) ( θ ω + = t A t V
c c

where A = amplitude of the carrier signal

c
ω = angular frequency

θ

= phase angle
Any one of these parameters may be varied in accordance with the baseband or message
signal.

The modulating signal is preserved in the envelope of amplitude modulated signal
only if
c m
V V < . There are three degrees of modulation depending upon the amplitude of the
message signal relative to the carrier amplitude.

Under modulation: In this case 1 <
a
m , i.e
c m
V V < . Here the envelope of amplitude
modulated signal does not reach the zero amplitude axis. Hence the message signal is fully
preserved in the envelope of the AM wave. This is known as under modulation. An envelope
detector can recover the message signal without any distortion.

Critical Modulation: In this case 1 =
a
m , i.e
c m
V V = . Here the envelope of the modulated
signal just reaches the zero amplitude axis. The message signal remains preserved. This is
known as critical modulation. In this case also the modulated signal can be recovered using an
envelope detector without any distortion.

Over Modulation: In this case 1 >
a
m , i.e.
c m
V V > . In this case the amplitude of modulating
signal is greater than carrier amplitude. Therefore a portion of the envelope of the modulated
signal crosses the zero axis. In this case, the envelope detector cannot recover the message
signal exactly from the envelope. Clipping is observed in the detected signal. This is called
envelope distortion. Due to this envelope detector provides distorted message signal.
Percentage of modulation is defined as 100 mod %
min max
min max
×
+
−
=
V V
V V
ulation . Generally it is
desirable to keep the percent of modulation high. For a given transmitter power, a high
percent of modulation will produce a stronger audio tone in the receiver.

DEMODUALTION:

Demodulation or detection is the process by which the modulating voltage is
recovered from the modulated signal. In an AM envelope, the carrier amplitude has
symmetrical variations in its positive and negative half cycles of modulating voltage. As the
envelope is symmetrical, the mean value of the detected current becomes zero. For detection
purpose, non linear devices are used which make the modulated wave unsymmetrical, either
by wiping (clipping) out any one half cycle or by making positive and negative half cycles
unequal.

PROCEDURE:

1. Turn on the trainer and adjust the oscilloscope trigger controls for stable display.
2. Measure the period of the carrier wave generated by XR2206 and calculate the
frequency.
3. Patch the circuit as shown in the circuit diagram. Connect 200Hz sine wave from
audio oscillator to input of the modulator i.e. to one end of the 100KΩ potentiometer
1
R .
Patch the output of XR2206 to one end of 10KΩ potentiometer
2
R . Keep
2
R to its
maximum position.
4. Connect the oscilloscope to the AM output.
5. With potentiometer
1
R at mid-range, measure the percentage of modulation and sketch the
AM output.
6. Adjust R
1
clockwise to 100% modulation. Observe and sketch the AM output and
calculate the percentage of modulation.
7. Adjust R
1
clockwise to observe over modulation and calculate the percentage of
modulation.
8. Connect the modulated signal output to the input of demodulator circuit in the
trainer.
9. Observe and note down the output waveforms at different stages and verify whether
this signal is a amplitude-scaled version of the message signal.

The XR2206 is a multi-purpose function generator chip. In this experiment, we use this IC to
generate a RF signal with frequency 100KHz. The period of this RF signal is determined by
the F µ 001 . capacitor and 10KΩ resistor. The frequency is therefore 100KHz. This frequency
is however affected by the tolerance of the resistor and the capacitor. In the AM transmitter
section, series modulation is used. Transistor Q
1
is the RF amplifier and Q
2
is the modulator.
The potentiometer R
1
varies the percentage of modulation. The diode detector is used for
demodulation of AM signal. The input to this circuit is the AM waveform. This signal is
applied to diode D
1
, which acts as a half-wave rectifier. The positive half cycles cause D
1
to
conduct, developing positive pulses across R
1
. D
1
cuts off the negative half cycles of the RF
input. This is sketched as shown. The center waveform shows the voltage developed across R
1

if S
1
is open. When S
1
is closed, C
1
is placed in parallel with R
1
. C
1
(0.01 µ F) quickly charges
through D
1
to the peak of each positive pulse. Between pulses, C1 attempts to discharge
through R
1
. However, the RC time constant is chosen so that C1 discharges only slightly. The
result is that the voltage across C
1
follows the envelope of the AM waveform. Thus, the
output looks like the upper envelope with a small amount of ripple. This ripple is eliminated
by the blocking capacitor C
2
(22 F µ ).

MODEL WAVEFORMS:

V
m
< V
c

V
m
= V
c

V
m
> V
c

DE MODULATED SIGNAL:

RESULT:

Thus the amplitude modulated waveform with different modulation and demodulation
indices are observed.

VIVA-VOCE:

1. Define AM and draw its spectrum?
2. Draw the phasor representation of an amplitude modulated wave?
3. Give the significance of modulation index.
4. What are the different degrees of modulation?
5. What are the limitations of square law modulator?
6. Compare linear and nonlinear modulators.
7. Compare base modulation and emitter modulation.
8. Explain how AM wave is detected.
9. Define detection process.
10. What are the different types of distortions that occur in an envelope detector? How
can they be eliminated?
11. What is the modulation used in radio broad casting.
12. What is IF frequency range in AM receiver?
13. What is meant by super heterodyne? What are the advantages?
14. Differences between TRF and super heterodyne receiver.
15. What are the characteristics of receiver?

REMARKS:

Signature of the Faculty.

Expt. No:
Date:

AM DSB - SC MODULATION & DE MODULATION

AIM: To observe DSB-SC amplitude modulated signal using balanced modulator and to
calculate the carrier suppression ratio for MC1496 IC.

Two important parameters of a communication system are transmitting power and
bandwidth. Hence in any communication system, saving of power and bandwidth is highly
desirable. In conventional AM, there is wastage in carrier power. In order to save the power in
amplitude modulation the carrier is suppressed, because it doesn’t contain any useful
information. This scheme is called as double side band suppressed carrier amplitude
modulation (DSB-SC-AM).

The DSBSC signal is given by V(t)
DSBSC
= V
m
(t)V
c
(t)

where V
m
(t) = message signal
V
c
(t) = carrier

Product modulator is used to generate DSB-SC-AM signals. There are different product
modulators and balanced modulator is one such modulator. In this modulator, a DSBSC wave
is generated by using two AM modulators arranged in a balanced configuration so as to
suppress the carrier wave.

IC BALANCED MODULATORS:

The circuit diagram shows an IC that has been specifically designed for use as
balanced modulators. The 1496 balanced modulator, which is manufactured by Motorola
National and Signetics uses a differential amplifier configuration. Its carrier suppression is
rated at a minimum of –5dB with a typical value of –65dB at 500 KHz.

DE MODULATOR:

The baseband signal m(t) can be uniquely recovered from a DSBSC wave s(t) by first
multiplying s(t) with a locally generated sine-wave and then low pass filtering the product.
The block diagram is shown below.

The local oscillator signal used for detection should be coherent or synchronized, in
both frequency and phase, with the carrier wave c(t) used in the product modulator for
DSBSC generation. This method of demodulation is known as coherent detection or
synchronous detection. Coherent detection is a special case of a more general demodulation
process using a local oscillator signal of the same frequency but with phase difference ϕ,
measured with respect to the carrier wave c(t).

In the above equation, the first term denotes a DSBSC wave with a carrier frequency
2fc, whereas the second term is proportional to the baseband signal m(t) . The first term can be
removed by using a low pass filter with cut-off frequency greater than W but less than 2fc -W,
i.e. fc > W . At the filter output the signal is given as

The demodulated signal V
0
(t) is proportional to m(t) when the phase error ϕ is constant. The
amplitude of the demodulated signal is maximum when ϕ = 0
0
and it is minimum or zero
when ϕ = 90
0
. The zero demodulated signal which occurs for ϕ =90
0
is described as the
quadrature null effect of the coherent detector. This phase error causes the detector output to
be attenuated by a factor cos ϕ. As long as the phase error is constant, the detector output
produces an undistorted version of the original baseband signal m(t). In practice, the phase
error ϕ varies randomly with time, due to random variations in the channel. Thus at the
detector output, the multiplying factor cos ϕ also varies randomly with time, which is
obviously undesirable. Thus at the receiver, additional circuitry must be provided to maintain
the local oscillator in perfect synchronism, in both frequency and phase, with the carrier wave
used to generate the DSBSC wave in the transmitter.

PROCEDURE:

MODULATOR:

1. Switch on the trainer. The trainer internally generates the message signal and the carrier
signal. The message signal is a sine signal with frequency 5 KHz and the carrier signal is
also a sine signal with frequency 100 KHz. The amplitudes of these signals can be varied
using the potentiometers.
2. Observe the message and carrier signals and check the frequency values.
3. Connect the RF signal generator output to the RF input of the balanced modulator and
observe the output. Adjust the RF potentiometer for minimum output and note down the
corresponding value as E
out carrier only
.
4. Now connect the AF signal generator output to the AF input of the balanced modulator
and observe the DSB-SC output. Also adjust the null adjustment control for a stable DSB-
SC output.
5. Now adjust for maximum output without producing clipping and note down the peak
output voltage as E
peak side bands
.
6. Calculate the carrier suppression in dB and sketch the DSB-SC wave.
7. Turn off the kit and disconnect the circuit.

1. Switch on the trainer and observe the output of the RF generator using CRO. The output
should be a sine wave of 100 KHz frequency. The amplitude can be varied using the
potentiometer.
2. Observe the output of the AF generator using CRO. The output should be a sine wave of
5 KHz frequency. The amplitude can be varied using the potentiometer.
3. Connect the RF and AF generator outputs to RF and AF inputs of the balanced modulator
respectively. Observe the output of the balanced modulator. Adjust the null adjustment
control to get a proper DSBSC wave.
4. Now connect the balanced modulator output to synchronous detector input. Also connect
the RF generator output to RF input of the synchronous detector.
5. Now observe the output of the synchronous detector circuit. The output should be a sine
wave with frequency 5 KHz.

TABULAR FORM:

Signal Type

Amplitude in volts

Frequency in hertzs

Carrier Signal

Modulating Signal

De modulated signal

DISCUSSION:

In this circuit, Colpitts oscillator using FET (BFWII) is used to generate an RF signal
of approximately 100 KHz frequency. Separate control is provided on the panel to adjust the
output amplitude. For AF signal generation, a fixed frequency (5 KHz) wein-bridge oscillator
using op-amp. IC TL084 is a FET input general-purpose op-amp. A separate control is
provided to vary the amplitude of the output signal. IC MC1496 is used as a balanced
modulator to generate DSBSC wave, which in turn is used as an input to the synchronous
detector.

MC 1496 is a monolithic IC and can be used up to 200MHz. A separate control called
null-adjust control is provided to adjust the carrier suppression. For synchronous detection
purpose, IC MC1496 is used. On board generated carrier is used as synchronous signal.

RESULT:

The DSB-SC amplitude modulated wave was observed and the carrier suppression
ratio was calculated using MC1496 and the demodulated signal was observed using synchronous
detector.

VIVA-VOCE:

1. Define AM?
2. Define DSB-SC system?
3. What is the working of Balanced Modulator?
4. Define Modulator and Demodulator?
5. Define coherent detector?
6. What are the different detectors used in
a) Critical modulation b) Under modulation c) Over modulation
7. What are the advantages and disadvantages of DSB-SC modulation?
8. What is a squelch circuit?
9. What are the different types of fading and solution?
10. Why SSB is not used for video broad casting.

Many instruments are available for the analysis of electrical signals in the time and
frequency domains. The traditional way of observing electrical signals is to view them in the
time domain using an oscilloscope. The time domain is used to recover relative timing and
phase information, which is needed to characterize electric circuit behaviour. However, not all
the circuits can be uniquely characterized from just time domain information. Circuit elements
such as amplifiers, oscillators, mixers, modulators, detectors and filters are best characterized
by their frequency response information. This frequency information is best obtained by
viewing electrical signals in the frequency domain. To display the frequency domain requires
a device that can discriminate between frequencies while measuring the power level at each.

One instrument which displays the frequency domain, is the spectrum analyzer. It
graphical displays voltage or power as a function of frequency on a CRT (Cathode Ray Tube).
In the time domain, all frequency components of a signal are seen summed together. In the
frequency domain, complex signals (i.e. signals composed of more than one frequency) are
separated into their frequency components, and the power level at each frequency is
displayed. The frequency domain is a graphical representation of signal amplitude as a
function of frequency. The frequency domain contains information not found in the time
domain and therefore, the spectrum analyzer has certain advantages compared with an
oscilloscope.

The analyzer is more sensitive to low level distortion than a scope. Sine waves may
look good in the time domain, but in the frequency domain, harmonic distortion can be seen.
The sensitivity and wide dynamic range of the spectrum analyzer is useful for measuring low-
level modulation. It can be used to measure AM, FM, and pulsed RF. The analyzer can be
used to measure carrier frequency, modulation frequency, modulation level, and modulation
distortion. Frequency conversion devices can be easily characterized. Such parameters as
conversion loss, isolation, and distortion are readily determined from the display. The
spectrum analyzer can be used to measure long and short term stability. Parameters such as
noise side bands an oscillator, residual FM of a source and frequency drift during warm up
can be measured using the spectrum analyzer's calibrated scans. Swept frequency
measurements are possible with the spectrum analyzer. These measurements are simplified by
using a tracking generator.

Spectrum Analyzer Block Diagram

Types of Spectrum Analyzers

There are two basic types of spectrum analyzers, swept-tuned and real-time analyzers.
The swept tuned analyzers are tuned by electrically sweeping them over their frequency
range. Therefore, the frequency components of a spectrum are sampled sequentially in time.
This enables periodic and random signals to be displayed, but makes it impossible to display
transient responses. Real-time analyzers, on the other hand, simultaneously display the
amplitude of all signals in the frequency range of the analyzer, hence the name real-time. This
preserves the time dependency between signals which permits phase information to be
displayed. Real-time analyzers are capable of displaying transient response as well as periodic
and random signals.

The swept-tuned analyzers are usually of the TRF (tuned radio frequency) or super
heterodyne type. A TRF analyzer consists of a band pass filter whose centre frequency is
tunable over a desired frequency range, a detector to produce vertical deflection on a CRT,
and a horizontal scan generator used to synchronize the tuned frequency to the CRT, and a
horizontal scan generator used to synchronize the tuned frequency to the CRT horizontal
deflection. It is a simple, inexpensive analyzer with wide frequency coverage, but lacks
resolution and sensitivity. Because TRF analyzers have a sweep filter they are limited in
sweep width depending on the frequency range (usually one decade or less). The resolution is
determined by the filter bandwidth, and since tunable filters do not usually have constant
bandwidth, is dependent on frequency.

The most common type of spectrum analyzer differs from the TRF spectrum analyzers
in that the spectrum is swept through a fixed band pass filter instead of sweeping the filter
through the spectrum. The analyzer is basically a narrowband receiver which is electronically
tuned in frequency by applying a saw-tooth voltage to the frequency control element of a
voltage tuned local oscillator. The same saw tooth voltage is simultaneously applied to the
horizontal deflection plates of the CRT. The output from the receiver is synchronously
applied to the vertical deflection plates of the CRT and a plot of amplitude versus frequency is
displayed The analyzer is tuned through its frequency range by varying the voltage on the LO
(Local Oscillator). The LO frequency is mixed with the input signal to produce an IF
(Intermediate Frequency) which can be detected and displayed. When the frequency
difference between the input signal and the LO frequency is equal to the IF frequency, then
there is a response on the analyzer.

The advantages of the super heterodyne technique are considerable. It obtains high
sensitivity through the use of IF amplifiers, and many decades in frequency can be tuned.
Also the resolution can be varied by changing the bandwidth of the IF filters. However, the
super heterodyne analyzer is not real-time and sweep rates must be consistent with the IF
filter time constant. A peak at the left edge of the CRT is sometimes called the "zero
Sweep
Generator
Sweep
Oscillato
r
Mixer Filter

Detector
To CRO
frequency indicator" or "local oscillator feed through". It occurs when the analyzer is tuned to
zero frequency, and the local oscillator passes directly through IF creating a peak on the CRT
even when no input signal is present. This effectively limits the lower tuning limit.

MODEL GRAPHS:

Illustrating the time domain (on the left) and frequency domain ( on the right characteristics of
deferent modulated waves produced by a single tone (a) Modulating (b) Carrier wave
(c) AM wave (d) DSB-SC wave (e) SSB wave with the upper side frequency transmitted (f)
SSB wave with the other side frequency transmitted).

PROCEDURE:

1. Connect the AM signal from the AM modulator kit to the spectrum analyzer.
2. Record the spectral lines and calculate the powers of carrier LSB and USB spectral lines.
3. Calculate the band width of a AM wave.
4. Connect the DSB-SC wave from the Balanced Modulator kit to the spectrum analyzer.
5. Record the spectral lines and calculate the side band powers.
6. Calculate the band width of DCS-SC wave.
OBSERVATIONS:

AM WAVE:

Message signal =

Carrier signal =

Lower Side Band (LSB) = f
c
- f
m

= _______

Carrier Frequency f
c
= _______

Upper Side Band (USB) = f
c
+ f
m

= _______

DSB-SC WAVE:

Message signal =

Carrier signal =

Lower Side Band (LSB) = f
c
- f
m

= _______

Upper Side Band (USB) = f
c
+ f
m

= _______

RESULT: Thus observed the AM and DSB-SC spectrums on the spectrum analyzer.

VIVA - VOCE:

1. Define spectrum analyzer.
2. Define AM and draw its spectrum.
3. Draw the phasor representation of an amplitude modulated wave.
4. Give the significance of modulation index of AM and FM.
5. What are the different degrees of modulation?
6. Define DSB-SC.
7. Give the significance of bandwidth of AM &FM.
8. Difference between time domain signal and frequency domain signal.
9. What is PSD?
10. Why 5KH
Z
in television as BW of modulation signal.

1. Make the connections as shown in the figure. Set the audio oscillator frequency to 1 KHz.
2. Increase the d.c. power supply voltage E
dc
to 2V to initially reverse bias the detector
output.
3. Now increase the function generator output voltage to 1 Vrms.
4. Decrease E
dc
from 2V in steps of 0.2V and measure the current Idc flowing through the
ammeter for each value of the d.c. voltage.
5. Bring the d.c. voltage E
dc
back to 2V. Repeat the experiment for the audio signal voltage of
2 V
rms
and 3 V
rms
. Plot the dc voltage Vs dc current characteristics of the detector with E
dc

The rectification efficiency of the diode for the specified RF voltage is calculated as,
Rectification efficiency =
=

Maximum depth of modulation at the input for which negative peak clipping can be avoided
is given by =
=

MODEL GRAPH:

RESULT: Thus studied the diode detector characteristics.

VIVA-VOCE:

1. What is the purpose of diode in diode detector circuit?
2. What are the disadvantages of simple diode detector circuit?
3. What are the advantages of practical diode detector?
4. What is the function of diode in diode detector?
5. Choice of local oscillator frequency.
6. Explain a) Image channel interference. b) Adjacent channel interference.
7. Define the term selectivity and sensitivity of a receiver?
8. What are the factors influencing the choice of intermediate frequency in receivers?

REMARKS:

Signature of the Faculty.

Expt. No:
Date:
PRE-EMPHASIS AND DE-EMPHASIS

AIM: To obtain the characteristics of pre-emphasis and de-emphasis circuits.

At the demodulator output, the noise power density rises parabolically with frequency.
Thus with rise in frequency, the power density decreases. Thus the noise is strongest in
frequency range where the signal is weakest. The high frequency components of the message
signal therefore suffer the most because of noise interference on the channel. This difficulty
can be avoided by using pre-emphasis and de-emphasis technique.

Since the magnitude of noise can’t be reduced, the only alternative is to increase the
magnitude of high frequency components of the modulating signal. The purpose of pre-
emphasis is to increase the magnitude of high frequency components of the modulating
signal. As a result, the SNR ratio is improved. A de-emphasis circuit is always used in
receiver circuit to restore relative magnitudes of different improvements in AF signal.

PROCEDURE:

PRE-EMPHASIS

1. Give the connections as per the circuit diagram.
2. Apply 100mv, 100Hz signal as input to the pre-emphasis network.
3. Keeping the input voltage constant at 100mv, vary the frequency of the signal in steps of
100Hz up to 15 KHz and note down the corresponding output voltage values.

DE-EMPHASIS

1. Give the connections as per the circuit diagram.
2. Apply 10V, 100Hz signal as an input to the de-emphasis network.
3. Keeping the input voltage constant at 100mv, vary the frequency of the signal in steps of
100Hz up to 15 KHz and note down the corresponding output voltage values.

TABULAR FORM:

PRE-EMPHASIS:

S.No Frequency(Hz) O/p voltage(V
0
) Gain in dB
20log(V
0
/V
i
)

DE-EMPHASIS:

S.No Frequency(Hz) O/p voltage(V
0
) Gain in dB
20log(V
0
/V
i
)

MODEL GRAPH:

Gain in dB

Pre-emphasis

Frequency

De- emphasis

RESULT: Thus the pre-emphasis and de-emphasis characteristics are studied.

VIVA -VOCE:

1. What is the need for pre-emphasis?
2. Explain the operation of pre-emphasis circuit.
3. Pre emphasis operation is similar to high pass filter with gain in pass band explain
how?
4. De emphasis operation is similar to low pass filter with attenuation in stop band,
Justify?
5. What is de-emphasis?
6. Draw the frequency response of a pre-emphasis circuit.
7. Draw the frequency response of a de-emphasis circuit.
8. Give the formula for the cutoff frequency of the pre-emphasis circuit.
9. What is the significance of the 3dB bandwidth.

REMARKS:

Signature of the Faculty.

Expt. No:
Date:
FREQUENCY MODULATION AND DEMODULATION

AIM: To generate the frequency modulated waveform with different modulation indices and
demodulate the same.

In the process of modulation, some characteristics of a high frequency (carrier) sine
wave is varied in accordance with instantaneous values of the information or modulating
signal.

FREQUENCY MODULATION:

Frequency modulation is that form of angle modulation in which the instantaneous
frequency ) (t f
i
is varied linearly with the base band signal m(t). The instantaneous frequency
of an FM signal is given by ) ( ) ( t m k f t f
f c i
+ = .
The term
c
f represents the frequency of the un-modulated carrier, and the constant
f
k
represents the frequency sensitivity of the modulator, expressed in Hz/volts (assuming m(t) is
a voltage waveform). Integrating the above equation with respect to time and multiplying the
result by π 2 , we get
∫
+ =
t
f c i
dt t m k t f t
0
) ( 2 2 ) ( π π θ . (Assuming that the angle of the
unmodulated carrier wave is zero at t = 0).
The frequency modulated wave is thus given as }] ) ( 2 2 cos[ ) (
0
∫
+ =
t
f c c
dt t m k t f A t s π π
The figure below shows the frequency modulated or FM wave form. The information
or modulating signal and the un modulated carrier is shown in figure a. With FM, the
modulating signal changes the frequency of the carrier rather than its amplitude. The resulting
frequency modulated waveform is shown in the figure b. At
0
T , the modulated waveform is at
its center frequency. As the modulating signal swings positive, the frequency of the carrier is
increased. The carrier reaches its maximum frequency when the modulating signal reaches its
maximum amplitude at time
1
T
.
At time T
2
, the modulating signal returns to 0 and the carrier
returns to its center frequency. After T
2
, the modulating signal swings negative. This forces
the carrier below its center frequency. The carrier again returns to its center frequency when
the modulating signal returns to 0 volts at time T
4
. Between times T
4
and T
8
, the modulating
signal repeats its cycle. As a result, the carrier is again shifted in frequency. It swings first
above and then below its center frequency. Notice that it returns to its center frequency each
time the modulating signal passes through 0 volts.

The carrier changes equally above and below its center frequency. The amount of
frequency change is called the frequency deviation. For example, let’s assume that a carrier
continuously swings form 100 MHz up to 100.1MHz and back to 100MHz. The frequency
deviation is +0.1MHz or +100 KHz. The rate of frequency deviation is determined by the
frequency of the modulating signal. For example, if the modulating signal is a 1 KHz audio
tone, the carrier will swing above and below its center frequency 1000 times each second. A
10 KHz audio tone will still cause the carrier to deviate +100 KHz, but this time at the rate of
10000 times each second. Thus, the frequency of the modulating signal determines the rate of
frequency deviation but not the amount of deviation.

The amount that the carrier deviates from its center frequency is determined by the
amplitude of the modulating signal. A high amplitude audio tone may cause a deviation of ±
100 KHz. A lower amplitude tone of the same frequency may cause a deviation of only ± 50
KHz.

Thus the frequency-modulated waveform has the following characteristics:
1. It is constant in amplitude but varies in frequency.
2. The rate of carrier deviation is the same as the frequency of the modulating signal.
3. The amount of carrier deviation is directly proportional to the amplitude of the
modulating signal.

MODULATION INDEX:

In AM, the degree of modulation is measured as a percentage form 0% to 100% or as
a modulation factor from 0 to 1. In angle modulation, the degree of modulation is measured
by the modulation index. The equation for modulation index is

m
d
f
f
m =

d
f = the frequency deviation

m
f = the modulating frequency
While the modulation factor in AM is limited to a decimal between 0 and 1, the modulation
index in angle modulation can reach quite high numerical values. Another measure of angle
modulation is the deviation ratio. This is the ratio of the maximum deviation to the maximum
audio frequency; thus, it is a total system measurement rather than the instantaneous
measurement of modulation index.

FM DEMODULATOR

The 565 phase locked loop is a general purpose circuit designed for highly linear F.M
demodulation. During lock, the average D.C level of the phase comparator output is directly
proportional to the frequency of the input signal. As the input frequency shifts, it is this output
signal which causes the VCO to shift its frequency to match that of the input. Consequently
the linearity of the phase comparator output with frequency is determined by voltage to
frequency transfer function of the VCO. Because of its unique and highly linear VCO, the 565
can lock to and track an input signal over a very wide range (typically 60%) with very high
linearity (typically within 0.5%).

PROCEDURE:

1. Turn on the FM trainer and patch the circuit as shown in the figure.
2. Connect 200Hz sine wave from the audio oscillator to the input of FM.
3. Connect the oscilloscope to the pin 2 of IC XR-2206 (output) and set the CRO time/cm
control to 2ms/cm and the vertical input to 1 v/cm. The CRO should display a sine wave
output. If not check the circuit for patching errors.
4. Turn potentiometer to mid range. The CRO should show a slightly blurred sine wave. This
graphically illustrates the frequency deviation of the FM output. It occurs because the
oscilloscope triggers each move at the same point on the display. However, since each
cycle has a slightly different frequency, the blurred display results.
5. Calculate the values of
max min
&T T for the blurred sine wave. Calculate the frequency
deviation using the formula
|
|
.
|

\
|
− = ∆
max min
1 1
T T
f .
6. Patch the FM output to the input of the demodulator.
7. Connect the CRO to pin 7 of the 565 phase locked loop. Set the time/cm control to
2ms/cm and the vertical input to 0.5V/cm. At this point, you may or may not observe the
audio output signal displayed on the CRO. You must adjust the 565 PLL to the correct
operating frequency. To do this, adjust R
2
until a sine wave output is displayed on the
oscilloscope. At this point, the VCO operating frequency is same as the input frequency.
The sine wave output is the error voltage required to keep the VCO locked on to the input
FM signal.
8. Note down the amplitude and check the frequency of the demodulated signal.
9. Now increase the amplitude of the input signal and repeat steps 5 - 8 for this input signal.
10. For a given input signal, adjust R1 and note the effect on the blurred output. R
1
controls
the amplitude of the message signal and thus changes the value of deviation.
11. Also by keeping the input signal amplitude constant, change the frequency of the input
signal and observe the blurred output. Since the input amplitude is constant, the deviation
should remain constant irrespective of the change in input frequency.
12. Turn off the trainer.

Thus the FM modulated and demodulated waveforms were observed and the
frequency deviation for different amplitudes of the input signal was calculated.

VIVA-VOCE:

1. Define frequency modulation.
2. Mention the advantages of indirect method of FM generation.
3. Define modulation index and frequency deviation of FM.
4. What are the advantages of FM?
5. What is narrow band FM?
6. Compare narrow band FM and wide band FM.
7. Differentiate FM and AM.
8. How FM wave can be converted into PM wave?
9. State the principle of reactance tube modulator.
10. What is the bandwidth of FM system?
11. What is the function of FM discriminator?
12. How does ratio detector differ from fosterseely discriminator?
13. What is meant by linear detector?
14. What are the drawbacks of slope detector?
15. Explain the circuit operation?

A Simple AGC is a system by means of which the overall gain of a radio receiver is
varied automatically with the changing strength of the received signal, to keep the output
substantially constant. The devices used in those stages are ones whose transconductance and
hence gain depends on the applied bias voltage or current. It may be noted that, for correct
AGC operation, this relationship between applied bias and transconductance need not to be
strictly linear, as long as transconductance drops significantly with increased bias. All modern
receivers are furnished with AGC, which enables tuning to stations of varying signal strengths
without appreciable change in the size of the output signal thus AGC "irons out" input signal
amplitude variations, and the gain control does not have to be re adjusted every time the
receiver is tuned from one station to another, except when the change in signal strength is
enormous.

In addition, AGC helps to smooth out the rapid fading which may occur with long-
distance short-wave reception and prevents the overloading of the last IF amplifier which
might otherwise have occurred.

BLOCK DIAGRAM:

DESCRIPTION:

1. RF Generator:
Colpitts oscillator using FET is used here to generate RF signal of 455 KHz frequency
as carrier signal in this Experiment. Adjustments for amplitude and frequency are provided on
the panel for ease of operation.

4. AM Modulator:
The modulator section illustrates the circuit of modulating amplifier employing a
transistor (BC107) as an active device in common emitter amplifier mode.R
1
and R
2
establish
a quiescent forward bias for the transistor. The modulating signal fed at the emitter section
causes the bias to increase or decrease in accordance with the modulating signal. R
4
is emitter
resistance and C
3
is by pass capacitor for carrier. Thus the carrier signal applied at the base
gets amplified more when the amplitude of the modulating signal is at its maximum and less
when the signal by the modulating signal output is amplitude-modulated signal. C
2
couples
the modulated signal to output of the Modulator.

a) 1
st
IF Amplifier:
Q
2
(8F 495C) acts as 1
st
IF Amplifier. The base of Q
2
is connected through R
5
(68KΩ)
to the detector output. R
6
(100Ω) and C
4
(47n) is decoupling filter for +B line. The base
potential depends on R
4
(220KΩ) base biasing resistor and the detector current supplied by
R
5
. The detector current is proportional to the signal strength received. This controls the bias
of Q
2
(SF495C) automatically to the signal received. This is called AGC. C
6
(4.7/16) is used
as base bias and AGC decoupling capacitor. The output of Q
2
is available .across the
secondary of L
8
(IF T2), the primary of which is tuned to if by the capacitor C
18
(2n 7). This
output is given to the base of Q
3
(SF 495D)
.
b) 2nd IF Amplifier:
Q
3
(SF 195C) acts as 2
nd
IF amplifier. The base bias for Q
3
is provided by R
7

(180KΩ), C
7
(47n) is used to keep the end 4 of L
8
(IFT2) at ground potential for IF signal.
The collector of Q3 is connected to the L9 (IFT3). L
9
contains 200pf capacitor inside across
the primary. The output of Q3 is available across the secondary of L9, the primary of which is
tuned by the internal 200pf capacitor, R
8
(220E), C
8
(47n) consists the decoupling circuit for
the collector supply of Q
3
. The output of 03 is coupled to detector diode 01 (OA 79).

c) Detector:
Modulated IF signal from the secondary of L
9
(IFT3) is fed to the detector diode 01.
01 rectifies the modulated IF signal & IF component of modulated signal is filtered by C8
(22n), R9 (680EO & C14 (22n). R9 is the detector load resistor. The detected signal (AF
signal) is given to the volume control P
2
(10k Log) though maximum audio output-limiting
resistor r21 (10k), It is also given to AGC circuit made of R5 (68k) and C6 (a.7/16).

d) AGC:
The sound received from the LS will depend on the strength of the signals received at
the antenna. The strength of the received signals can vary widely due to fading. This will
cause variations in sound which can be annoying. Moreover, the strength of signals can also
be too large in close vicinity of MW transmitters causing overloading of the 2" IF amplifier.
Automatic gain control (AGC) is used to minimize the variations in sound with changes in
signal strength & to prevent overloading. The operation of AGC depends on the fact that the
gain obtained from any transistor depend on its collector current & becomes less when the
collector current is reduced to cut off (or increased to saturation) For AGC, DC voltage
obtained from the detection of IF a signal is applied to the 1st amplifier transistor base in such
a way that an increase in this voltage reduces the gain of the transistor. The result is that when
the strength of the incoming signal increases, the DC voltage also increases and this tends to
reduce the gain of the amplifier thus not permitting the output to Change much. Here R5 (68k)
& C6 (4.7/16) performs this function. C6 (4.7/16) is the AGC decoupling capacitor to by pass
any AF signals and keep the bias steady.

PROCEDURE:

1. As the circuit is already wired you just have to trace the circuit according to the circuit
diagram given above.
2. Connect the trainer to the mains and switch on the power supply.
3. Measures the output voltages of the regulated power supply circuit i.e. +12v and -12v,
+6@150mA.
4. Observe outputs of RF and AF signal generator using CRO, note that RF voltage is
approximately 50mVpp of 455 KHz frequency and AF voltage is 5Vpp of1 KHz frequency.
5. Now vary the amplitude of AF signal and observe the AM wave at output, note the
percentage of modulation for different values of AF signal.
% Modulation= (Emax -Emin) /(Emax+Emin) × 100
6. Now adjust the modulation index to 30% by varying the amplitudes of RF & AF signals
simultaneously.
7. Connect AM output to the input of AGC and also to the CRO channel -1
8. Connect AGC link to the feedback network through OA79 diode
9. Now connect CRO channel - 2 at output. The detected audio signal of 1 KHz will be
observed.
10. Calculate the voltage gain by measuring the amplitude of output signal (Vo) waveform,
using Formula A = Vo/V i
11. Now vary input level of 455 KHz IF signal and observe detected 1 KHz audio signal with
and Without AGC link. The output will be distorted when AGC link removed i.e. there is no
AGC action.
12. This explains AGC effect in Radio circuit.

1. Define is AGC?
2. Classify AGC?
3. What are the applications of AGC?
4. What are the functions of IF Amplifier?
5. What are the functions of RF Amplifier?
6. What is the need for AGC?
7. What are the drawbacks of AGC and solution?
REMARKS:

A band limited signal which has no spectral components above f
m
(modulating signal
frequency) is sampled at the regular time intervals with a sampling period less than or equal to
1/2fm (Ts<= 1/2fm).

2. Frequency domain statement.

A band limited signal which has no spectral components above f
m
is reconstructed by
collecting all the samples at the rate of 2f
m
samples / second.

The ideal sampled signal can be represented as

∞
g
δ
(t)= ∑g(nT
s
) δ(t-nT
s
)

n=- ∞
The fourier transform of an ideal sampled signal can be written as

∞
G
δ
(f)= 1/T
s
∑G(f-n/T
s
)

n=- ∞
For reconstruction of original signal from the sampled values ,we go for interpolation formula
with the sinc function sinc(2fmt).

∞

g(t) = ∑g(n/2fm)sinc(2f
m
t-n) - ∞ ≤ t ≤ ∞

n=- ∞
The sampling rate of 2f
m
per second, for a band limited signal of bandwidth f
m
Hz is called
Nyquist rate. The original signal can be recovered by passing the sequence of samples through
an ideal Low-Pass Filter called the reconstruction filter of band width f
m
Hz.

1. Connect the circuit as shown in the figure (A).
2. Apply a sine wave of 1KH
Z
frequency having 2V p-p amplitude.
3. Apply a sampling pulse of 2V p-p amplitude and 4KHz frequency.
4. Observe the output in the C.R.O.
5. Plot the graph of sampled output

Recovery:

1. Connect the circuit as shown in the figure (B).
2. Apply the sampled output to the circuit.
3. Observe the output at recovered output and compare with the input.
4. Increase the frequency of the analog input to the sampler and observe the effect.
5. Plot the graph of reconstructed output.

RESULT:

The analog signals are sampled and reconstructed and the results are plotted on the
graph. Thus verified Shannon’s sampling theorem.

VIVA-VOCE:

1. State the Shannon’s sampling theorem.
2. What is Nyquist rate?
3. What is meant by Aliasing?
4. What are the effects of Aliasing?
5. How to avoid Aliasing effect?
6. What are the various Sampling techniques?
7. Explain various sampling circuits?
8. Why you need a hold circuit?

REMARKS:

Signature of the Faculty.

Expt. No:
Date:

PULSE AMPLITUDE MODULATION AND DEMODULATION

AIM: To construct a Pulse Amplitude Modulation and Demodulation circuit and observe the
output waveforms.

In analog modulation, one of the characteristics of the carrier like amplitude,
frequency, or phase is continuously varied in accordance with the amplitude of modulating
signal. However, in pulse modulation, a pulse train is taken as carrier. In this the pulse
amplitude or duration or position is varied in accordance with the amplitude of the sampled
modulating signal.

Pulse Amplitude Modulation circuit uses a 4016 integrated circuit CMOS switch.
Basically, it is a FET logic switch. When the sampling pulse goes positive, the switch closes
and the modulating input appears across R and output. When sampling pulse drops to zero,
the switch opens and the output is zero. The circuit provides dual-polarity PAM. However,
single-polarity PAM can be achieved by adding R
1
, R
2
.These resistors form a voltage divider
that adds a DC level to the input signal. The result is that the input AC wave now varies
around a positive DC reference rather than a zero-volt reference.
The Demodulator for PAM signal is merely a low-pass filter. It removes the sampling
signal and its harmonics, and passes the original modulating signal. However, the roll-off of
the filter must be steep enough to pass the highest modulating frequency and to fully attenuate
the lowest sampling frequency component. That is the filter’s cutoff must fall well within the
guard band of the particular PAM system.

MODEL WAVE FORMS:

MODULATING SIGNAL

DEMODULATED SIGNAL

PULSE AMPLITUDE
MODULATED SIGNAL
CARRIER PULSE TRAIN
PROCEDURE:

MODULATION:

1. Connect the circuit as per the circuit diagram.
2. Apply analog input that is sine wave and pulse input from the function
generator.
3. Observe the output of Pulse Amplitude Modulation wave on C.R.O screen.

DEMODULATION:

1. The output of the modulator is fed to the base of the transistor (BC108) in the
demodulation circuit.
2. Demodulation output is taken across the collector of the Transistor (BC 108).
3. Sketch and observe the waveforms.

RESULT:

Thus constructed the Pulse amplitude modulation and demodulation circuit and
modulated and demodulated waveforms are observed.

VIVA VOCE:

1. Classify modulation techniques?
2. Define PAM.
3. What are the advantages of PAM?
4. What are the demodulation methods for the flat-top sampled signal?
5. Compare natural and flat-top sampling with the help of waveforms.
6. What is meant by aperture effect?
7. What is meant by ideally or instantaneous sampled PAM.
8. What are the disadvantages of PAM?
9. Applications of PAM.

REMARKS:

Signature of the Faculty.

Expt. No:
Date:

PWM, PPM -MODULATION AND DEMODULATION

AIM: To study the PWM, PPM modulation and demodulation process and observe the
corresponding waveforms.

In Amplitude and angle modulation, some characteristics of the carrier amplitude,
frequency, or phase is continuously varied in accordance with the modulating information.
However, in pulse modulation, a small sample is made of the modulating signal and then a
pulse is transmitted. In this case, some characteristics of the pulse is varied in accordance with
the sample of the modulating signal. The sample is actually a measure of the modulating
signal at a specific time.

In PWM, the samples of message signal are used to vary the duration of the individual
pulses. So this type of modulation is referred to as pulse duration modulation or pulse length
modulation. The pulse width is proportional to the amplitude of the analog signal. The
modulating wave may vary the time of occurrence of leading edge, or both edges of pulse.

A typical PWM waveform can be generated using a 555 timer. When the timer is
connected in the monostable mode and triggered with a continuous pulse train, the output
pulse width can be modulated by a signal applied to pin no.5, .i.e. modulating signal.

PPM:

Pulse position modulation may be obtained very simply from pulse width modulation.
The trailing edges of PWM pulses are, in fact, position modulated. The method of obtaining
PPM from PWM is thus accomplished by getting rid of the leading edges and bodies of the
PWM pulses. This is surprisingly easy to achieve the train of pulses obtained in PWM are
differentiated and another pulse train results. This has positive going narrow pulses
corresponding to leading edges and negative going pulses corresponding to trailing edges. If
the position corresponding to the trailing edge of an un modulated pulse is counted as zero
displacement, then the other trailing edges will arrive earlier or later.

The time displacement is proportional to the instantaneous value of the signal voltage.
The leading edges are removed with a diode clipper or rectifier and the remaining pulses are
inverted These pulses are now given to a filter to broaden the width of the unmodulated
pulses. This is the required PPM wave.

When PPM is demodulated in the receiver, it is again converted into PWM .This is
done with a flip-flop bistable multivibrator. One input of the multivibrator receives trigger
pulses from a local generator which is synchronized by trigger pulses in the transmitter. The
PPM pulses to other base of the flip-flop and switch that stage ON. The resulting PWM pulse
train is then demodulated.

1. Now connect the output terminal of the modulated signal to the input of the
demodulation circuit.
2. Observe the demodulated output waveform, which is the replica of the input. Plot the
graph.

PPM:

MODULATION:

1. Connect the circuit as per the circuit diagram.
2. Apply a Sinusoidal wave of frequency 1 KHz at the modulating signal input (pin no.5)
3. With applying the modulating signal observe the reference clock.
4. Observe the output wave forms at pin no 3.And plot the graph.

DEMODULATION:

1. Now connect the output terminal of the modulated signal to the input of the
demodulation circuit.
2. Observe the demodulated output waveform, which is the replica of the input. Plot the
graph.
CIRCUIT DIAGRAM:

1. What are the different types of Pulse time modulation systems?
2. Define PDM?
3. What are the advantages of PWM?
4. What are the disadvantages of PWM?
5. What is the BW of PWM?
6. What are the methods to generate PWM?
7. Define PPM?
8. What are the advantages of PPM?
9. What are the disadvantages of PPM?
10. What is the BW of PPM?
11. How to generate PPM?
12. Applications of PDM and PPM?

REMARKS:

Signature of the Faculty.

Expt. No:
Date:

PHASE LOCKED LOOP

AIM: To compare the theoretical and practical values of capture range and lock range of
phase locked loop.

A phase locked loop is basically a closed loop system designed to lock the output
frequency and phase to the frequency and phase of an input signal. It is commonly
abbreviated as PLL. PLL’s are used in applications such as frequency synthesis, frequency
modulation/demodulation, AM detection, tracking filters, FSK demodulator, tone detector etc.
The block diagram of PLL is as shown below:

The phase detector compares the input frequency
i
f with the feedback frequency
o
f
and generates an output signal which is a function of the difference between the phases of the
Lowpass
Filter
Error
Amplifier
Phase
Detector
VCO
Input
Input
Phase
comparat
or

VCO output
VCO
Amplifier
Phase
Detector
C
2

+V -V
R
1

C
1

+Vc
c
3.6 kΩ
Low pass filter
1
2
3
4
5
8 9
6
7
10
two input signals. The output signal of the phase detector is a dc voltage. The output of the
phase detector is applied to a low-pass filter to remove high frequency noise from the dc
voltage. The output of low pass filter without high frequency noise is often referred to as error
voltage or control voltage for VCO. When control voltage is zero, VCO is in free running
mode and its output frequency is called as center frequency
o
f . The non-zero control voltage
results in a shift in the VCO frequency from its free-running frequency,
o
f to a frequency f ,
given by
C V o
K K f f + =
where
V
K is the voltage to frequency transfer coefficient of the VCO. The error or
control voltage is same as the input signal frequency. Once the two frequencies are same, the
circuit is said to be locked. In locked condition, phase detector generates a constant dc level
which is required to shift the output frequency of VCO from centre frequency to the input
frequency. Once locked, PLL tracks the frequency changes of the input signal. Thus, a PLL
goes through three states: free running, capture and phase lock.

Lock Range: When PLL is in lock, it can track frequency changes in the incoming signal.
The range of frequencies over which the PLL can maintain lock with the incoming signal is
called the lock range or tracking range of the PLL.

Capture Range: The range of frequencies over which the PLL can acquire lock with an input
signal is called the capture range. The centre frequency of the PLL is determined by the free-
running frequency of the VCO and is given as Hz
C R
f
o
1 1
4
2 . 1
=
where
1
R and
1
C are an external resistor and a capacitor connected to pins 8 and 9,
respectively. The values of
1
R and
1
C are adjusted such that the free running frequency will
be at the centre of the input frequency range. The value of
1
R is restricted from 2kΩ to20kΩ,
but the capacitor can have any value. A capacitor
2
C is connected between pin7 and the
positive supply pin 10 forms a first order low pass filter with an internal resistance of 3.6k Ω.
The value of filter capacitor should be large enough to eliminate possible oscillations in the
VCO voltage. The lock range and capture range for IC 565 PLL are given by the following
equations: Hz
V
f
f
o
L
8
± =
where
o
f is the free-running frequency of VCO in Hz and ) ( ) ( V V V − − + = volts.

Hz
C
f
f
L
L
2
1
2
3
10 ) 6 . 3 ( 2
(
¸
(

¸

± =
π
Where
2
C is in farads.

As shown in the block diagram, the phase locked feedback loop is not internally connected.
Therefore, it is necessary to connect output of VCO (pin 4) to the phase comparator input (pin
5), externally. Lock range increases with an increase in input voltage but decreases with
increase in supply voltage. The two inputs (pin 2 and pin 3) to the phase detector allows direct
coupling of an input signal, provided that there is no dc voltage difference between the pins
and the dc resistances seen from pins 2 and 3 are equal. A reference voltage at pin 6 is
approximately equal to the dc voltage of the demodulated output at pin 7.
PROCEDURE:

1. Connect the circuit as per the circuit diagram on the breadboard.
2. Without giving input signal, find out the output signal frequency, which is called free
running frequency,
o
F .
3. Now apply 1V, 1 KHz sinusoidal signal as input and slowly increase the input frequency
and note down the corresponding output frequency.
4. When input and output frequencies are equal, then note down it as
1
F . Now increase the
input frequency slowly and the output frequency will also follow the input frequency. This
follow up will continue until a certain frequency point
2
F . Note down the value of
2
F .
Continue to increase the input frequency and then the output frequency will be back to
o
F .
5. Now decrease the input frequency slowly and at one point input and output frequencies
will be equal. Note down this point as
3
F .
6. Continue to decrease the input frequency. The output frequency will also follow once
again, this follow up continues up to
4
F . Note down this frequency value and decrease the
input frequency further. Then the output frequency will once again back to
o
F only.
7. Calculate the theoretical and practical values of free-running frequency, lock range and
capture range and compare them.

OBSERVATIONS:

Free-running frequency = F
0

Lock Range = F
3
-F
4

Capture Range = F
2
-F
1

THEORITICAL CALCULATIONS:

Lock range

F
4
F
1
F
0
F
2
F
3

Capture range

Free running frequency, F
0
=

= = 3kΩ

Lock range, F
L
= = = ±1.2kH
Z
= 2.4 kH
z

Capture range, F
C
= ±

= ± = ±103 H
Z
= 206 H
Z

RESULT:

Thus the theoretical and practical values of lock range and capture range for PLL are
calculated and compared.

VIVA-VOCE:

1. What are the applications of PLL?
2. What is a PLL?
3. What is a VCO?
4. Define the lock range of a PLL.
5. Define the capture range of PLL.
6. Give the expression for free running frequency f
0
of a PLL.
7. What is meant by free running frequency of a PLL?
8. Give the formulae for the lock range and capture range of the PLL.
9. What are the applications of PLL?